Air and gas streams often carry particulate material therein. In many instances, it is desirable to remove some or all of the particulate material from the gas flow stream. For example, air intake streams to engines for motorized vehicles or power generation equipment, gas streams directed to gas turbines, and air streams to various combustion furnaces, often include particulate material therein. The particulate material, should it reach the internal workings of the various mechanisms involved, can cause substantial damage. It is therefore preferred to remove the particulate material from the gas flow upstream of the engine, turbine, furnace or other equipment involved.
In other instances, production gases or off gases from industrial processes may contain particulate material therein, for example those generated by the process. Before such gases can be, or should be, discharged through various downstream equipment and/or to the atmosphere, it may be desirable to obtain substantial removal of particulate material from those streams.
A variety of air filter or gas filter arrangements have been developed for particulate removal. For reasons that will be apparent from the following descriptions, improvements have been desired for arrangements developed to serve this purpose.
A general understanding of some of the basic principles and problems of filter design can be understood by consideration of the following types of systems: a paper filter; a pleated paper filter; and, a constant density depth filter. Each of these types of systems is known, and each has been utilized.
Consider first a paper element, comprising a porous paper filter oriented perpendicularly to a gas stream having particulate material entrained therein. The filter paper selected will typically be one permeable to the gas flow, but of sufficiently fine porosity to inhibit the passage of particles no greater than a selected size therethrough. A simple, planar, filter construction made from such a material could in operation be oriented completely across the gas flow stream, for example between a source of air and an intake manifold for an engine. As the gases pass through the filter paper, the upstream side of the filter paper will receive thereagainst selected sized particles in the gas stream. The filter will act to remove the particles from the gas stream. The particles are collected as a dust cake, on the upstream side of the paper filter.
A simple filter design such as that described above is subject to at least two major types of problems. First, a relatively simple flaw, i.e. rupture of the paper, results in complete failure of the system, and thus lack of protection of downstream equipment. Secondly, particulate material will rapidly build up on the upstream side of the filter, as a thin dust cake or layer, eventually substantially occluding the filter to the passage of gas therethrough. Thus, such a filter would be expected to have a relatively short lifetime, if utilized in an arrangement involving in the passage of large amounts of gas therethrough, with substantial amounts of particulate material above the "selected size" therein; "selected size" in this context meaning the size at or above which a particle is stopped by, or collects within, the filter.
The filter lifetime, of course, would be expected to be related to the surface area of the paper filter, the rate of gas flow through the system, and the concentration of particles in the carrier stream. For any given system, the "lifetime" of a filter is typically defined according to a selected limiting pressure drop across the filter. That is, for any given application, the filter will have reached its lifetime of reasonable use when the pressure buildup across the filter has reached some defined level for that application.
An alternative design to that described above is a pleated paper filter. The arrangement of the filter paper in a pleated configuration generally increases the surface area of filter media provided within a given cross-sectional area or volume of space. It will also tend to increase the strength of the system. Thus, the operating lifetime of the filter is increased, due to the increase of surface area for entrainment of particulate material thereagainst. However, pleated paper media is still a surface loaded filter media. As a thin layer of particulate material collects on the upstream surface of the filter element, the filter will still tend to become occluded. Thus, the lifetime of such a filter is still relatively short, in applications. In addition, the system is again subject to significant problems should a minor flaw or rupture develop in the paper element.
It is noted that in many applications, the gas stream to be filtered can be expected to have particulate material of a variety of sizes therein, and/or the equipment can be expected to be subjected to varying gas flow streams with respect to particulate content. Consider, for example, a filter arrangement designed for utilization in motorized vehicles. It will be preferred that the filter arrangements utilized for such vehicles be capable of filtering out particles ranging from submicron sizes up to 100 microns. For example, vehicles utilized in off-road circumstances, at construction sites or at other sites (country roads perhaps) where a lot of dirt is carried in the air, can be expected to encounter gas streams carrying a substantial percent of about 10 to 100 micron material. Most of the air which passes through the air filter of an over-the-highway truck or automobile, when the vehicle does not encounter dust storms or construction sites, generally carries relatively little particulate material above about 5 microns in size, but does carry a substantial portion of submicron to 5 micron sized materials. A city bus, on the other hand, principally encounters only submicron sized carbon particles in the gases passing into the filter thereof. However, even city buses can be expected to at least occasionally encounter air having larger particles therein.
In general, filters designed for vehicles should preferably be capable of providing substantial protection to the engine for particles throughout a size range of submicron to 100 microns, regardless of what are expected to be the preponderant working conditions of any specific vehicle. That is, such arrangements should be developed such that they do not rapidly occlude, under any of a wide variety of conditions likely to be encountered during the lifetime of the vehicle. Such is true, of course, for any filter system. However, with respect to vehicles, the problem is exacerbated by the fact that the vehicle moves from environment to environment, and thus can be expected to encounter a relatively wide variety of conditions. A "flexible" arrangement is preferred at least in part so that one construction of filter can be put to use in a relatively wide variety of applications.
Consider again the paper filter and pleated filter arrangements described above. A paper filter will relatively rapidly occlude, i.e. reach its lifetime through buildup of filter cake and generation of limiting differential. Thus, a given filter paper construction would not be expected to be a very effective system for filtering air under a wide variety of applications, especially with expectation of a relatively long lifetime. In addition, as explained above, paper filter arrangements do not in general provide good protection, in the event of failure. That is, even a minor rupture or tear can result in a nearly complete system failure.
In many applications, an alternative type of filter, generally referred to as a "depth" filter, is available. A typical depth filter is a thick layer or web of fibrous material referred to as "depth media." Depth media is generally defined in terms of its porosity, density or percent solids content. Typically, it is defined in terms of its solids content per unit volume, for example a 2-3% solidity media would be a depth media mat of fibers arranged such that approximately 2-3% of the overall volume comprises the fibrous material (solids), the remainder being air or gas space. Another useful parameter for defining depth media is fiber diameter. If percent solidity is held constant, but fiber diameter is reduced, pore size reduces; i.e. the filter becomes more efficient and will more effectively trap smaller particles.
A typical conventional depth media filter is a deep, relatively constant (or uniform) density, media, i.e. a system in which the solidity of the depth media remains substantially constant throughout its thickness. By "substantially constant" in this context, it is meant that only relatively minor fluctuations in density, if any, are found throughout the depth of the media. Such fluctuations, for example, may result from a slight compression of an outer engaged surface, by a container in which the filter is positioned.
A problem with constant or uniform solidity depth media systems, is that they are not readily adapted for efficient filtering under circumstances in which air or gas flow with varying populations of particle sizes are likely to be encountered. If the percent solidity of the depth media is sufficiently high, relatively large particles will tend to collect in only the outermost or most upstream portions of the media, leading to inefficient utilization of the overall media depth. That is, under such circumstances the particles of solids (especially larger ones) tend to "load" on the front end or upstream end of the media, and do not penetrate very deeply. This leads to premature occlusion or a short lifetime. By "premature" in this context, it is meant that although the depth media volume is large enough for much greater "loading" of solids, occlusion results because the load is heavily biased toward the front end, and results in blockage (and early pressure differential increase).
If, on the other hand, relatively low density depth media is utilized, a greater percent of its volume will tend to be loaded or filled by larger particles, with time. This may occur, for example, through redistribution as particle agglomerates initially formed in more upstream regions, break up and redistribute inwardly. Thus, at the "lifetime" or "limiting pressure differential" load would be more evenly distributed through the media depth (although completely uniform distribution is unlikely). However, very large and very small particles would be more likely to have passed completely through such a system.
From the description, it will be apparent that constant density depth media is not particularly well suited for circumstances in which either: the population of particle sizes within the air flow extends over a relatively wide range; and/or, the air filter is likely to encounter a variety of air streams (conditions) presenting therein a variety of particle size distributions.
Very low density depth media, on the order of about 1-3%, and more typically about 1-2% solidity, is sometimes referred to as "high loft" media. Such media has been utilized as filter media in HVAC filters (heat, ventilation, air conditioning).
The term "load" and variants thereof as used above and referred to herein in this context, refers to amount or location of entrainment or entrapment of particles by the depth media filter.
As explained above, as the density (i.e. percent solidity) of the depth media is increased, under constant load conditions, after use the filter will tend to include a greater load toward the upstream side. Should the load conditions comprise air having a variety of particle sizes therein, or should the filter need to operate under a variety of conditions of use, no single density depth media has, in the past, been as effective as may be desired, as a filter. That is, for any given percent solids depth media, the load pattern will differ depending upon the particle size distribution within the air or gas stream to be filtered. Thus, while the filter depth could be optimized for one particular particle size, it might not be sufficient for operation under a variety of conditions or with gas having a variety of particle sizes therein.